‘You haven’t quite got me. I don’t say we must find him. It will be enough to suggest him. All we have to do is to show that some other person had a motive for Madame’s death, and could have murdered her and carried out the plot against Felix. A doubt would then arise as to which of the two was guilty, and, if that doubt was strong enough, Felix would get the benefit of it.’ ‘But that makes our problem no easier. The difficulty still lies in the finding of this other person.’ ‘We can only try; it may lead to something. Our first question then is: If Felix is innocent, who might be guilty?’ There was silence for several seconds, then Heppenstall spoke again. ‘Who, perhaps I should say, is least unlikely to be guilty?’ ‘I think there can be only one answer to that,’ returned Clifford. ‘In the very nature of the case a certain suspicion must attach to Boirac. But the police were fully alive to that. From all we hear, they went into it thoroughly and came to the conclusion he was innocent.’ ‘It depended on an alibi. But you know as well as I do alibis can be faked.’ ‘Undoubtedly, but they concluded this one wasn’t. We don’t know the exact details, but it seems to have been fully tested.’ ‘At all events, from the information available, I think we may assume that if Felix is innocent, Boirac is guilty. There is no suggestion of any third party being involved. If, then, we can show that Boirac had a motive for the crime, and that he could have committed it and made the plant, that’s all we want. We have not to prove him guilty.’ ‘I suppose that is so. Then our next point is: What might have been Boirac’s motive?’ ‘That’s not hard to find. If Boirac found his wife was carrying on with Felix, it might explain his desire to kill her.’ ‘Yes, and it would give a two-fold reason for his working for Felix’s conviction; first, self-defence by shifting over the suspicion, and, second, revenge on the man who had spoilt his home.’ ‘Quite. I think a plausible motive might be built up. Next let us ask, When was the body put in the cask?’ ‘The police say in London, because there was no opportunity elsewhere.’ ‘Yes, and to me it seems a quite sound deduction. Now, if that is true, it follows that if Boirac killed his wife, he must have travelled here to do it.’ ‘But the alibi?’ ‘Leave the alibi for a moment. Our defence must be that Boirac followed his wife to London and murdered her there. Now can we suggest possible details? He would arrive at his house on that Sunday morning and find his wife gone, and a letter from her saying she had eloped with Felix. What, then, would he do?’ Clifford leaned forward to stir the fire. ‘I have thought over that,’ he said somewhat hesitatingly, ‘and I have worked out a possible theory. It is, of course, pure guesswork, but it fits a number of the facts.’ ‘Let’s hear it. Naturally our theories at present can only be guesswork.’